Vitrification does not affect birth weight: lessons from the oocyte donation model.

RESEARCH QUESTION: Is embryo cryopreservation a cause of high birth weight and large for gestational age (LGA) in singletons resulting from vitrified-warmed embryo transfer? DESIGN: Retrospective cohort study evaluating 670 oocyte recipients who underwent fresh (367 cycles) or vitrified-warmed embryo transfer (303 cycles) at Instituto Bernabeu between July 2017 and March 2019. All single blastocyst transfers carried out in an artificial cycle that resulted in a singleton live birth were included. RESULTS: Maternal age (42.21 ± 4.45; 42.79 ± 3.83; P = 0.519), body mass index (23.34 ± 3.69; 23.80 ± 3.78; P = 0.075), gestational age (38.96 ± 1.97; 38.77 ± 2.15; P = 0.207), maternal smoking (10.8%; 13.0%; P = 0.475), gestational diabetes (4.9%; 4.3% P = 0.854), preeclampsia (2.7%; 5.6%; P = 0.074), hypertensive disorders (3.3%; 2.3%; P = 0.494), maternal parity (multiparous 18.5%; 14.5%; P = 0.177) and liveborn gender (female 44.5%; 48.8%; P = 0.276) were not significantly different between fresh or vitrified-warmed groups. Endometrial thickness was significantly higher in the fresh versus vitrified-warmed group (8.83 ± 1.73 versus 8.57 ± 1.59; P = 0.035, respectively). Oocyte donor height was similar between the fresh versus vitrified-warmed group (163.22 ± 5.88 versus 164.27 ± 6.66 cm; P = 0.057, respectively). Mean birth weight was not significantly different (3239.21 ± 550.43; 3224.56 ± 570.83; adjusted P = 0.058). No differences were observed in macrosomia (7.1%; 6.3%; adjusted OR 0.857, 95% CI 0.314 to 2.340, P = 0.764), LGA (6.0%; 6.7%; adjusted OR 0.450, 95% CI 0.176 to 1.149, P = 0.095), pre-term birth (10.9%; 9.0% adjusted P = 0.997), very pre-term birth (0.8%; 1.3%; adjusted P = 1.000), extremely pre-term birth (0%; 1.0%; adjusted P = 0.998); underweight (10.0%; 7.0%; adjusted P = 0.050); very low weight (0.6; 1.1%; adjusted P = 1.000) and small for gestational age (1.9%; 0.7%; adjusted P = 0.974) between fresh or vitrified-warmed groups. CONCLUSION: This study eliminates potential confounders that might influence fetal growth and demonstrates that embryo vitrification and warming procedures do not affect birth weight.

Early epigenetic reprogramming in fertilized, cloned, and parthenogenetic embryos.

Despite ongoing research in a number of species, the efficiency of embryo production by nuclear transfer remains low. Incomplete epigenetic reprogramming of the nucleus introduced in the recipient oocyte is one factor proposed to limit the success of this technique. Nonetheless, knowledge of reprogramming factors has increased-thanks to comparative studies on reprogramming of the paternal genome brought by sperm on fertilization-and will be reviewed here. Another valuable model of reprogramming is the one obtained in the absence of sperm fertilization through artificial activation-the parthenote-and will also be introduced. Altogether the objective of this review is to have a better understanding on the mechanisms responsible for the resistance to reprogramming, not only because it could improve embryonic development but also as it could benefit therapeutic reprogramming research.